The present invention relates to a method and apparatus for controlling fuel vapor, a method and apparatus for diagnosing a fuel vapor control apparatus, and a method and apparatus for controlling an air-fuel ratio. Particularly, the present invention relates to a technique for controlling the fuel vapor, a technique for diagnosing the fuel vapor control apparatus, and a technique for controlling the airfuel ratio accompanying the fuel vapor control, of an engine mounted on a vehicle.
Conventionally, there has been known a fuel vapor control apparatus which comprises a canister with an adsorbent of an activated carbon or the like for adsorbing and collecting evaporation gas generated in a fuel tank of a vehicle, a purge piping for supplying purged air from the canister to an intake air system of the engine utilizing the intake negative pressure of the engine, and a purge control valve mounted on the purge piping for controlling a purge flow rate, for controlling the opening of a purge control valve based on a target purge flow rate set according to the operating conditions of the engine (refer for example to Japanese Patent Application Unexamined Publication No. 11-182360).
In order to treat a large quantity of evaporation gas adsorbed to the canister, the opening area of the purge control valve in its fully opened state must be large, and for this purpose, the purge control valve must have a large size. However, there occurred a problem that a large-sized valve generally has a low resolution for flow rate control, and therefore, the control accuracy of the purge flow rate is deteriorated.
Further, there is a need to diagnose whether the purge control valve is malfunctioning or not, however, in the conventional constitution wherein a pressure sensor is mounted for detecting the pressure inside the purge piping for diagnosis, the pressure sensor to be used only for diagnosis is needed. Therefore, the conventional diagnosis system was expensive.
Moreover, if the evaporation gas is supplied from the canister to the engine, it means that excessive fuel is supplied with respect to a new air quantity. Therefore, the air-fuel ratio is shifted to a rich side. In order to solve this problem, conventionally, a correction is made to the fuel injection quantity so as to restrain the air-fuel ratio from being shifted to a rich side by the supply of evaporation gas during purge, and released such correction when the purge is ceased.
However, the supply of evaporation gas to the engine is not cut off immediately after closing the purge control valve, but evaporation gas is still being supplied to the engine, though the amount thereof gradually being reduced even after the purge control valve has been closed. Therefore, conventionally, even if an air-fuel ratio feedback control is being performed, there is a possibility that the air-fuel ratio is greatly and transitionally varied immediately after closing the purge control valve, which resulted in problems such as torque fluctuation or emission deterioration.
The present invention aims to solve the above-mentioned problems. The object of the invention is to provide a fuel vapor control apparatus and control method which enables to control a purge flow rate with high accuracy while enabling to treat a large quantity of evaporation gas.
Further object of the present invention is to provide a diagnosis apparatus and diagnosis method of a fuel vapor control apparatus that is capable of diagnosing malfunction of purge control valves without using a pressure sensor.
Moreover, the object of the invention is to provide an air-fuel ratio control apparatus and control method that enables to restrain a transitional fluctuation of the air-fuel ratio when ceasing the purge.
In order to achieve the above objects, the present invention comprises a pair of purge pipings arranged in parallel, and purge control valves having different flow rate sizes mounted to said pair of purge pipings respectively, wherein a flow rate of the purge control valve having a larger flow rate size is varied in a step mode, and a flow rate of the smaller purge control valve is controlled to be equal to or smaller than the flow rate varied in the step mode, so as to control the flow rate to a target flow rate. Thereby, a large flow rate control is enabled while securing accurate control of the flow rate.
Further, the invention comprises a pair of purge pipings arranged in parallel, and purge control valves mounted to the pair of purge pipings, respectively, wherein, when the target purge flow rate is equal to or below a threshold value, the flow rate is controlled by opening only one purge control valve, and when the target purge flow rate exceeds the threshold value, the opening of the purge control valve which has been open controlled is fixed, and the other purge control valve which has been close controlled is opened. Thereby the flow rate which could not be obtained by controlling the one purge control valve to a fully-opened position is compensated for by performing the open control of the other purge control valve.
Here, in a constitution where two purge control valves are equipped and the ratio of opening time for each purge control valve is controlled, the opening timing of the two valves may be mutually diverged so as to restrain pulsation of the purge flow rate.
Moreover, in a system equipped with two purge control valves, an air-fuel ratio obtained when open controlling one valve to a reference opening area and an airfuel ratio obtained when open controlling the other valve to a reference opening area are compared with each other. When deviation of the air-fuel ratios exceeds a reference value, it is determined that either of the two valves is not controlled to the reference opening area, and a malfunction determination signal is output.
Moreover, when a malfunction is determined by the above diagnosis, the two purge control valves are both controlled to a fully-closed position, and an air-fuel ratio during such state is detected. The detected air-fuel ratio is compared with the air-fuel ratio obtained when open controlling the valves to the reference opening area, in order to determine whether each of the purge control valves is actually opened equivalent to the reference opening area. Even further, when it is judged that each of the purge control valves is not opened equivalent to the reference opening area, determination is made on whether the valve is fixed to have a smaller opening area than the reference opening area (close-fixed state), or whether the valve is fixed to have a larger opening area than the reference opening area (open-fixed state).
On the other hand, the fluctuation of the air-fuel ratio directly after ceasing the purge is restrained by correcting a fuel quantity to be supplied to the engine based on estimation of an evaporation gas quantity to be supplied to the engine after ceasing the purge.
The evaporation gas quantity to be supplied to the engine after ceasing the purge may be estimated as characteristics varying with time. Even further, a reference model of the varying characteristics of the quantity may be equipped to estimate the evaporation gas quantity provided that the evaporation gas to be supplied to the engine varies following the reference model.
Other objects and aspects of the present invention will become apparent in the following description explaining the embodiments of the present invention with reference to the accompanied drawings.